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Nucleosynthesis and Aggregation

My question is: how do asteroids become rich in a particular metal? Surely the platinum would have been dispersed evenly throughout the cloud of gas emerging from the super/hypernova. If so, how does it aggregate again?

Let's ignore the misleading story from RT (the asteroid doesn't contain 5.4 trillion dollars worth of platinum), and simply address the question of differentiation.
One thing worth noting on this topic is that many of the heavy atoms that formed our asteroids and comets were in grains, not gasses, by the time our pre-solar nebula began to condense, and the distribution wasn't as uniform as you are assuming.
This IS an interesting topic, and I've only read a few papers on it over the years, so I imagine that some of our members will be able to add a lot more detail.

Looks like you're the only one who feels confident enough to comment antoniseb.

I guess I'm trying to trace the formation of the asteroid from the supernova. I can accept that platinum (say) atoms are not formed evenly throughout the explosion, but even if they form into grains, they will surely disperse over the millennia in the great gas clouds from which the solar system forms. I just can't see them staying close enough to each other to aggregate in such concentrations in the one asteroid.

I would expect the differentiation to occur during the rough and dirty action after the primordial nebula has started to collapse and form the proto-Sun and the planetesimals. I can imagine that in any given clump of collapsing gas laced with dust, the heavy and chemically inert grains of platinum and similar metals would settle toward the center, while the chemically reactive and lighter components would stay in the envelope. I hereby defer to experts who are studying this professionally to go into more theoretical detail.

I can imagine that in any given clump of collapsing gas laced with
dust, the heavy and chemically inert grains of platinum and similar
metals would settle toward the center, while the chemically reactive
and lighter components would stay in the envelope.

In that case, I'd expect the biggest difference to be between
the central star and the rest of the collapsing cloud. If the
differentiation occurs later, after the star becomes very bright
and windy, I can see materials with different physical properties
piling up at different distances from the center. But I also wonder
to what extent magnetic fields in the disk play a role, especially
considering what chornedsnorkack pointed out, that platinum
likes to associate with iron.

The normal way differentiation occurs is that a very large body forms via its own growing gravity, and that gravity also makes the body quite hot so it melts. Then heavier elements in the liquid object will sink to the center, like the iron core of the Earth. Then something smacks into the large body and splinters it into asteroids, which now have very different compositions depending on how close to the center of the object they came from. This is how we get iron meteorites, for example. So the question is then, does high platinum content occur because the original body had a lot of platinum originally, or did the differentiation process separate the platinum from the iron based on their different density. It could be either really, but what's clear is that this asteroid is going to be mostly iron, so calling it a "platinum asteroid" is highly misleading. (If it was really mostly platinum, it would contain more platinum than is contained in the entire Earth, and would cause the platinum market to fall until platinum was no more valuable than any other metal.)

What I don't understand is, it seems to me the entire concept of mining asteroids is bogus. The price of precious metals is pegged to the cost of mining them, because there are always metals not getting mined because the price of the metal would make it impossible to make a profit. If the price goes up, more reserves make sense to mine, and if the price goes down, less do. No doubt there is already a lot of platinum on Earth that is simply not being mined because it would be too costly to do so. Thus, the amount of platinum in some asteroid is never what matters, what matters is the cost to extract it. So it would only ever make sense to mine an asteroid if it was cheaper to do so than to mine the same metal from reserves that are currently too expensive to go after on Earth. Mining in space never seemed to me to be a particularly cheap way to do it-- even after you've spent the ghastly sum to pull the asteroid into Earth orbit, or something, you then have to send constant missions to it to break off more pieces. How is that ever going to be cheaper than digging a hole? People always seem to focus on how much of the metal is present in the asteroid, when they should be concentrating on the cost to extract an amount comparable to what is now mined each year.

A "siderophile" is an element that easily forms metallic bonds
with iron, under the kind of physical conditions Ken described
as occurring inside a planetoid large enough to melt due at least
in part to its own gravity. That seems pretty big, to me. Big enough
that it would require a real whopper of a collision to break it apart
and scatter the fragments. Such collisions would have happened
many times over the history of the Solar System. But I also know
that vast numbers of marble-sized bits of rocky material were
partially melted early in the Solar System's development. They
are the chondrules in chondritic meteoroids. The chondrules are
mostly stone, but there are usually metallic flecks mixed in with
them. I've handled meteorites which were sawed open to reveal
the chondrules inside. They were obviously compacted together
at very high temperature and pressure -- but not high enough to
melt the chondrules completely. Not enough for gravitational
differentiation within a large planetesimal with internal melting.
These chondrites are of course not the iron-nickel asteroids
that have relatively large amounts of platinum, but they do show
very interesting differentiation, some of which is obvious even
to a casual visual inspection of the cut surface.

How is that ever going to be cheaper than digging a hole? People always seem to focus on how much of the metal is present in the asteroid, when they should be concentrating on the cost to extract an amount comparable to what is now mined each year.

Indeed. When people have talked about mining asteroids or the moon, I've often pointed out that even if there were stacks of already-refined gold (or platinum) bars sitting in neat piles, it still wouldn't be cost effective to go get them.

Most solar nebula models could not deal effectively with chondrules because fast cooling is required. One model that worked includes super sonic flows that caused heating of ~3000K and allowed quick cooling. I assume the heavier elements would " take the heat" along these fronts with strong differentation results. The inner accretion disk, closest to the protosun, was a hot zone, thus another place that ejected light elements, no doubt. The inner regions of the disk also had different composition than the outer region, especially the disk's "atmosphere". Here should help. Google Bo Reipurth to get his pdf formation newsletters, too.

Indeed. When people have talked about mining asteroids or the moon, I've often pointed out that even if there were stacks of already-refined gold (or platinum) bars sitting in neat piles, it still wouldn't be cost effective to go get them.

Yeah, it's the same reaction I get when people talk about colonizing Mars, or the ocean floor. These ideas sound very cool, but it has to make basic economic sense or it's never going to happen. You can't decree that people should live in a manner that costs more than it produces. Earth isn't going to subsidy asteroid mining or Mars colonization just to be able to achieve those things, like some modern Egyptian pyramid.

Most solar nebula models could not deal effectively with chondrules because fast cooling is required. One model that worked includes super sonic flows that caused heating of ~3000K and allowed quick cooling. I assume the heavier elements would " take the heat" along these fronts with strong differentation results. The inner accretion disk, closest to the protosun, was a hot zone, thus another place that ejected light elements, no doubt. The inner regions of the disk also had different composition than the outer region, especially the disk's "atmosphere". Here should help. Google Bo Reipurth to get his pdf formation newsletters, too.

Yes, that's a good point, that compositions of smaller grains can be affected by their temperature of formation. So the first question is, did an asteroid with high platinum content form by aggregating lots of grains the formed with high platinum content due to their temperature, or did it differentiate in the strong gravity of a much larger body. It could even be both processes at work, I really don't know.

Then there's the point that Pt comes from merging neutron stars, so is their contribution like adding cream to coffee but with little or no stirring or will must elements contributed become nearly homogenous? I have more questions than answers but disks are certainly an area of active research.

Then there's the point that Pt comes from merging neutron stars, so is their contribution like adding cream to coffee but with little or no stirring or will must elements contributed become nearly homogenous?

There's a lot of stirring there, though. But yeah, the nucleosynthesis history of the gas can play a role too.

What? Huh???
A "siderophile" is an element that easily forms metallic bonds
with iron,

And platinum, unlike iron, is a strong siderophile. Which is my point.
Iron easily forms chemical bonds with itself and goes to iron meteors and planetary cores. But iron is a weak siderophile: iron also readily forms chemical bonds with oxygen and sulphur, and a lot of iron is in rocks.
Platinum, unlike iron, is a strong siderophile: platinum is much less inclined to bond with oxygen or sulphur than iron is.

What I don't understand is, it seems to me the entire concept of mining asteroids is bogus. The price of precious metals is pegged to the cost of mining them, because there are always metals not getting mined because the price of the metal would make it impossible to make a profit. If the price goes up, more reserves make sense to mine, and if the price goes down, less do. No doubt there is already a lot of platinum on Earth that is simply not being mined because it would be too costly to do so. Thus, the amount of platinum in some asteroid is never what matters, what matters is the cost to extract it. So it would only ever make sense to mine an asteroid if it was cheaper to do so than to mine the same metal from reserves that are currently too expensive to go after on Earth. Mining in space never seemed to me to be a particularly cheap way to do it-- even after you've spent the ghastly sum to pull the asteroid into Earth orbit, or something, you then have to send constant missions to it to break off more pieces. How is that ever going to be cheaper than digging a hole? People always seem to focus on how much of the metal is present in the asteroid, when they should be concentrating on the cost to extract an amount comparable to what is now mined each year.

Originally Posted by Grey

Indeed. When people have talked about mining asteroids or the moon, I've often pointed out that even if there were stacks of already-refined gold (or platinum) bars sitting in neat piles, it still wouldn't be cost effective to go get them.

I could not agree more with you both. Mining asteroids? We live on an asteroid! Thus far we have drilled perhaps drilled 12 kilometers into the crust. I suspect we have some way to go before we have to resort to space rubble millions of kilometers away.

Yeah, it's the same reaction I get when people talk about colonizing Mars, or the ocean floor. These ideas sound very cool, but it has to make basic economic sense or it's never going to happen. You can't decree that people should live in a manner that costs more than it produces. Earth isn't going to subsidy asteroid mining or Mars colonization just to be able to achieve those things, like some modern Egyptian pyramid.

With Mars there's also the small matter of proving the concept. We can't even live independently on Antarctica. When we can, we'll still be a long way from doing it on Mars. Ask every budding Martian colonizer if they're willing to spend the rest of their lives at the South Pole.

With Mars there's also the small matter of proving the concept. We can't even live independently on Antarctica. When we can, we'll still be a long way from doing it on Mars. Ask every budding Martian colonizer if they're willing to spend the rest of their lives at the South Pole.

Yeah, I see it as similar to when a child asks, "can we have a puppy as a pet?" I always say, "no, we cannot have a puppy as a pet because puppies grow up. Ask me if we can have a dog!" Most anyone would love the idea of being a colonist on Mars for a few months. What excitement that would be! But we aren't looking for colonists for a few months, we're looking for colonists for the rest of their lives-- very different issue! (And good point about living on an asteroid. Yes our asteroid might have lower concentrations of various things, but that must surely be made up for by the ease of access!)

For a several years, until recently, I felt the most "doable" way to
get a human on Mars would be a one-way trip for one person.
In the impossible situation that it became apparent that I was an
acceptable candidate and no others were volunteering, I'm pretty
sure I would have taken that trip. I'm too old now, as I just turned
65 a few hours ago. I still think a one-way trip for one person is
a good plan, but other people don't generally seem to be thinking
about it that way. On the other hand, someone did just put a car
into heliocentric orbit, so maybe the idea isn't so far off at that.
An awful lot of trips are one-way trips, many intentionally so.